1. Field of the Invention
The present invention relates to a dielectric element, and more specifically, it relates to a dielectric element such as a capacitor element employing an oxide-based dielectric film.
2. Description of the Prior Art
Deep study is recently made on a ferroelectric memory as a nonvolatile memory having a high speed and requiring low power consumption. FIGS. 29 and 30 are sectional views showing representative structures of conventional ferroelectric memories.
In the structure shown in FIG. 29, an isolation oxide film 101, a well region 103 for a MOS transistor 102, a source region 104, a source electrode 105 connected with the source region 104, a gate electrode 106, a drain region 107 and an interlayer isolation film 114 are formed on an Si substrate 100. An oxide-based dielectric capacitor 113 is connected to the drain region 107 through a plug 109.
In the structure shown in FIG. 30, an isolation oxide film 101, a well region 103 for a MOS transistor 102, a source region 104, a source electrode 105 connected with the source region 104, a gate electrode 106, a drain region 107, a drain electrode 108 connected with the drain region 107 and an interlayer isolation film 114 are formed on an Si substrate 100. An oxide-based dielectric capacitor 113 is connected to the gate electrode 106 through a plug 109. The structure shown in FIG. 30 is referred to as an FET-type ferroelectric memory.
In each of the structures shown in FIGS. 29 and 30, the oxide-based dielectric capacitor 113 is formed by a lower electrode 110, an oxide-based dielectric film 111 and an upper electrode 112. The lower electrode 110 is connected with the plug 109 made of polycrystalline silicon (poly-Si) or tungsten (W). The oxide-based dielectric film 111 of PbZrXTi1xe2x88x92XO3 (PZT) or SrBi2Ta2O9 (SBT) serving as a ferroelectric film is formed on the lower electrode 110. The upper electrode 112 is formed on the oxide-based dielectric film 111. In particular, iridium (Ir), platinum (Pt) or a material containing such a component is widely employed as the material for the lower electrode 110. This is because this material has low reactivity with the oxide-based dielectric film 111 or excellent high-temperature resistance. The upper electrode 112 is also made of a material such as iridium (Ir) or platinum (Pt), similarly to the lower electrode 110. In each of the structures shown in FIGS. 29 and 30, the upper electrode 112 is formed by an Ir film.
Also in a dynamic random access memory (DRAM), the capacitor size is recently reduced following refinement of cells and hence a capacitor structure employing an oxide-based dielectric film of BaXSr1xe2x88x92XTiO3(BST) or the like having a high dielectric constant is required. The capacitor structure of this DRAM is similar to that shown in FIG. 29.
However, self orientation of Ir or Pt is so strong that crystal grains exhibit a columnar structure when annealed. In this case, grain boundaries align in a direction perpendicular to the substrate. In annealing performed in a high-temperature oxygen atmosphere for sintering the oxide-based dielectric film forming a capacitor insulator film, therefore, oxygen diffuses along the grain boundaries. Thus, poly-Si or W forming an electrode such as a plug is oxidized to form an oxide film. Consequently, the capacitor characteristics are deteriorated or bad influence is exerted on preparation of the capacitor element.
When the plug 109 is prepared from poly-Si and partially oxidized in the element structure shown in FIG. 29, for example, a silicon oxide film is formed between the lower electrode 110 and the plug 109. In this case, the silicon oxide film serves as a capacitor insulator film and is serially connected to the oxide-based dielectric capacitor 113. When capacitors are serially connected, a bias applied thereto is divided in inverse proportion to the capacitance of each capacitor. The dielectric constant of an oxide-based dielectric film is generally several 10 to several 100 times that of a silicon oxide film, and hence the capacitance of the oxide-based dielectric capacitor 113 is increased.
Therefore, a bias applied in the state serially connecting the oxide-based dielectric capacitor 113 with the silicon oxide capacitor is not much divided to the oxide-based dielectric capacitor 113. In the case of a ferroelectric memory having the oxide-based dielectric film 111 of a ferroelectric film, for example, its inverted polarization value is reduced to disadvantageously deteriorate the memory characteristics. In the case of a DRAM having the oxide-based dielectric film 111 of a high dielectric film, its charging quantity is reduced to disadvantageously deteriorate the memory characteristics.
When made of W in place of the aforementioned poly-Si in the element structure shown in FIG. 29, the plug 109 is partially oxidized to form a tungsten oxide film. In this case, film separation results from volume expansion caused by forming the tungsten oxide film, leading to such a problem that it is difficult to prepare a high-quality capacitor element.
In addition, oxygen diffuses outward from the oxide-based dielectric film 111 along the grain boundaries of Ir or Pt forming the lower electrode 110 or the upper electrode 112, to disadvantageously deteriorate the characteristics such as the polarization characteristic of the oxide-based dielectric film 111 itself.
In the element structure shown in FIG. 29, the Ir film forming the upper electrode 112 is inconveniently oxidized when the oxide-based dielectric film 111 is annealed in an oxygen atmosphere to be sintered. FIGS. 31 and 32 are schematic sectional views for illustrating problems of the prior art.
When the Ir film forming the upper electrode 112 is oxidized in the annealing performed in the oxygen atmosphere for sintering the oxide-based dielectric film 111 as hereinabove described, a gigantic hillock (projection) 112a is readily formed on the surface of the upper electrode 112, as shown in FIG. 31 or 32. When such a hillock 112a is formed, a plate line 116a is disadvantageously disconnected as shown in FIG. 31. Or, an upper wire 118 formed on a plate wire 116b through an interlayer isolation film 117 is disadvantageously short-circuited to the plate wire 116b, as shown in FIG. 32.
When oxidized, the Ir film forming the upper electrode 112 causes compositional change to inconveniently result in stress change of the Ir film. Therefore, the ferroelectric characteristics also disadvantageously tend to change.
An object of the present invention is to provide a dielectric element having excellent characteristics by suppressing oxidation of an electrode.
Another object of the present invention is to suppress deterioration of the characteristics of an oxide-based dielectric film in the aforementioned dielectric element.
Still another object of the present invention is to provide a dielectric element capable of inhibiting the surface of an upper electrode from formation of a hillock (projection) by suppressing oxidation of the upper electrode.
A further object of the present invention is to suppress stress change resulting from compositional change of an upper electrode material.
A dielectric element according to an aspect of the present invention comprises an insulator film including an oxide-based dielectric film and an electrode including a first conductor film containing at least a metal and silicon. The aforementioned metal includes at least one metal selected from a group consisting of Ir, Pt, Ru, Re, Ni, Co and Mo. According to the present invention, the dielectric element is a wide concept including not only a capacitor element but also another element employing a dielectric material.
In the dielectric element according to this aspect, the first conductor film serves as a barrier film for stopping diffusion of oxygen due to the aforementioned structure. Thus, oxygen can be effectively inhibited from diffusing along grain boundaries of the electrode in heat treatment for sintering the oxide-based dielectric film. Therefore, a conductive material located under the electrode can be inhibited from oxidation. Thus, deterioration of memory characteristics can be suppressed and film separation can be prevented in the case of a capacitor element, for example. Consequently, an element having excellent characteristics can be formed.
In the dielectric element according to the aforementioned aspect, the first conductor film preferably further contains nitrogen. Thus, the function of the first conductor film for stopping diffusion of oxygen can be further improved. The metal (M) forming the dielectric element according to the aforementioned aspect hardly forms a nitride in general, or is stabilized in a state of Mxc3x97N (xxe2x89xa72) when forming a nitride. When such a metal is bonded with silicon (Si) and nitrogen (N), the metal (M) is more readily bonded with Si than with N while N is readily bonded with Si. Therefore, the Mxe2x80x94Sixe2x80x94N film conceivably has a structure obtained by embedding Sixe2x80x94N in metal silicide (Mxe2x80x94Si). Thus, the Mxe2x80x94Sixe2x80x94N film can conceivably have oxygen diffusion stoppability of the silicon nitride (Sixe2x80x94N) film and conductivity of the metal silicide (Mxe2x80x94Si) at the same time. Consequently, the Mxe2x80x94Sixe2x80x94N film can further improve the function of the first conductor film for stopping diffusion of oxygen.
In the aforementioned case, the metal forming the first conductor film is preferably iridium (Ir). When iridium is employed as the metal forming the first conductor film, the first conductor film can serve as the barrier film for stopping diffusion of oxygen. In this case, the first conductor film may be a conductor film containing iridium and silicon, or may be a conductor film containing iridium, silicon and nitrogen. The first conductor film may be formed by a multilayer structure of a conductor film containing iridium and silicon and a conductor film containing iridium, silicon and nitrogen. Thus, the conductor film containing iridium, silicon and nitrogen can keep high oxygen diffusion stoppability while the conductor film containing iridium and silicon can form a barrier film reduced in resistance.
In this case, the conductor film containing iridium, silicon and nitrogen is preferably arranged on the side of the oxide-based dielectric film. Thus, the first conductor film can more effectively stop diffusion of oxygen from the oxide-based dielectric film. Therefore, deterioration of the characteristics of the oxide-based dielectric film itself can be suppressed.
In the aforementioned case, the first conductor film is preferably arranged between a conductive material and the insulator film. Thus, the first conductor film can effectively inhibit oxygen from diffusing into the conductive material from the insulating material. In this case, the conductive material is preferably converted to an insulating material when oxidized, and the first conductor film and the insulator film are preferably successively formed on the conductive material. Thus, the first conductor film can effectively inhibit oxygen from diffusing into the conductive material from the insulating material, thereby suppressing oxidation of the conductive material. In this case, further, the conductive material preferably includes either a polycrystalline silicon plug or a tungsten plug. When employing a polycrystalline silicon plug or a tungsten plug as the conductive material, oxidation of the polycrystalline silicon plug or the tungsten plug is suppressed. Thus, a generally employed technique of forming a polycrystalline silicon or tungsten plug can be applied as such with no problem.
The dielectric element according to the aforementioned aspect preferably further comprises a conductive crystal film arranged between the first conductor film and the insulator film. Thus, the first conductor film can stop diffusion of oxygen while the conductive crystal film can form an insulator film consisting of an oxide-based dielectric film having excellent characteristics such as a polarization characteristic.
In this case, the conductive crystal film is preferably a metal film containing at least one metal selected from a group consisting of Pt, Ir, Ru and Re. Thus, the conductive crystal film consisting of the aforementioned metal film can form an insulator film consisting of an oxide-based dielectric film having excellent characteristics such as a polarization characteristic. In this case, the first conductor film preferably contains Pt, silicon and nitrogen, and the conductive crystal film is preferably a metal film consisting of Pt. Thus, the first conductor film containing Pt, silicon and nitrogen can more effectively stop diffusion of oxygen while the conductive crystal film consisting of the metal film of Pt can form an insulator film consisting of an oxide-based dielectric film having excellent characteristics such as a polarization characteristic.
In the aforementioned case, the conductive crystal film may be a metal oxide film containing at least one metal selected from a group consisting of Pt, Ir, Ru and Re. Thus, the conductive crystal film consisting of the aforementioned metal oxide film can form an insulator film consisting of an oxide-based dielectric film having excellent characteristics such as a polarization characteristic.
In the dielectric element according to the aforementioned aspect, the electrode including the first conductor film is preferably an upper electrode. When so formed as to include the first conductor film containing at least the metal and silicon having an excellent barrier property against oxygen diffusion, the upper electrode can be effectively inhibited from oxidation. Thus, the surface of the upper electrode can be inhibited from formation of a hillock (projection) resulting from oxidation of the upper electrode. Consequently, disconnection of wires or short-circuiting across the wires can be suppressed. Further, the material for the upper electrode can be inhibited from compositional change resulting from oxidation of the upper electrode. Thus, stress change of the upper electrode material can be suppressed, thereby suppressing change of the element characteristics. In this case, the first conductor film preferably further contains nitrogen. Thus, the function of the first conductor film for stopping diffusion of oxygen can be further improved.
In this case, further, the first conductor film preferably contains Ir, silicon and nitrogen. Thus, high oxygen diffusion stoppability can be implemented by employing the first conductor film containing Ir, silicon and nitrogen. Therefore, the first conductor film (upper electrode) can be effectively inhibited from oxidation.
In the aforementioned case, the upper electrode preferably includes a plurality of layers, and at least the uppermost layer of the upper electrode is preferably formed by the first conductor film. Thus, oxidation of the outermost surface of the upper electrode can be suppressed by forming at least the uppermost layer of the upper electrode by the first conductor film. In this case, the upper electrode is preferably formed by a multilayer structure of the first conductor film, containing Ir, silicon and nitrogen, forming the uppermost layer and a second conductor film containing Ir. Thus, it is possible to reduce the resistance of the upper electrode with the second conductor film containing Ir while suppressing oxidation of the upper electrode with the first conductor film containing Ir, silicon and nitrogen.
A dielectric element according to another aspect of the present invention comprises an insulator film including an oxide-based dielectric film and an upper electrode including a first conductor film containing TaN. According to this aspect, the upper electrode can be effectively inhibited from oxidation when so formed as to include the first conductor film containing TaN having an excellent barrier property against diffusion of oxygen. Thus, the surface of the upper electrode can be inhibited from formation of a hillock (projection) resulting from oxidation of the upper electrode. Consequently, disconnection of wires and short-circuiting across the wires can be suppressed. Further, compositional change of the upper electrode material resulting from oxidation of the upper electrode can be suppressed thereby suppressing stress change of the upper electrode material.